Calculating rate
rate of reaction = change in concentration of reactant or product divided by the time it takes for the change to take place.
As reactions proceed fewer collisions take place per second and so the reaction slows down.
Remember that the rate of change can be calculated using the formula for the slope of a line in coordinate geometry, i.e. the difference in the y-values divided by the difference in the x-values.
*hence rate is higher (reaction faster) at the beginning.